1. Field of the Invention
The present invention relates to a process for retorting hydrocarbon-containing solids, such as oil shale, in a combined fluidized-entrained bed.
2. Description of the Prior Art
Vast natural deposits of shale in Colorado, Utah and Wyoming contain appreciable quantities of organic matter which decomposes upon pyrolysis to yield oil, hydrocarbon gases and residual carbon. The organic matter or kerogen content of said deposits has been estimated to be equivalent to approximately 4 trillion barrels of oil. As a result of the dwindling supplies of petroleum and natural gas, extensive research efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
In principle, the retorting of shale and other similar hydrocarbon-containing solids simply comprises heating the solids to an elevated temperature and recovering the vapors evolved. However, as medium-grade oil shale yields approximately 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation. The choice of a particular retorting method must therefore take into consideration the raw and spent materials handling expense, as well as product yield and process requirements.
Process heat requirements may be supplied either directly or indirectly. Directly heated retorting processes rely upon the combustion of fuel in the present of the oil shale to provide sufficient heat for retorting. Such processes result in lower product yields due to unavoidable combustion of some of the product and dilution of the product stream with the products of combustion. Indirectly heated retorting processes, however, generally use a separate furnace or equivalent vessel in which a solid or gaseous heat carrier medium is heated. The hot heat carrier is subsequently mixed with the hydrocarbon-containing solids to provide process heat, thus resulting in higher yields while avoiding dilution of the retort product with combustion products, but at the expense of additional materials handling. The indirectly heated retort systems which process large shale or which use a gaseous heat transfer medium generally have lower throughputs per retort volume than the systems wherein smaller shale is processes or solid heat carriers are used.
In essentially all above-ground processes for the retorting of shale, the shale is first crushed to reduce the size of the shale to aid in materials handling and to reduce the time required for retorting. Many of the prior art processes, typically those processes which use moving beds, cannot tolerate excessive amounts of shale fines whereas other processes, such as the entrained bed retorts, require that all of the shale processed be of relatively small particle size, and still other processes, such as those using fluidized beds, require the shale to be of uniform size as well as being relatively small. Unfortunately, crushing operations have little or no control over the breadth of the resultant particle size distribution, as this is primarily a function of the rock properties. Thus, classification of the crushed shale to obtain the proper size distribution is normally required prior to retorting in most of the existing prior art processes and, in the absence of multiple processing schemes, a portion of the shale must be discarded.
In certain indirectly heated prior art retorts the hot heat carrier and shale are mechanically mixed in a horizontally inclined vessel. This mechanical mixing often results in high-temperature zones conducive to undesirable thermal cracking and/or low-temperature zones which result in incomplete retorting. Furthermore, as solids gravitate to the lower portion of the vessel, stripping the retorted shale with gas is inefficient and results in lower product yields due to readsorption of a portion of the evolved hydrocarbons by the retorted solids.
Prior art fluidized bed retorts have the advantages of uniform mixing and excellent solids-to-solids contacting over the mechanically mixed retorts; however, there is little control over the individual particle residence time. Thus, in such processes partially retorted material is necessarily removed with the retorted solids, leading to either costly separation and recycle of partially retorted materials, lowered product yields, or use of larger retort volumes. Furthermore, the gross mixing attained in such retorts results in poor stripping and readsorption of the product by the retorted solids. It must also be noted that it is very difficult to maintain a conventional stable fluidized bed of shale without extensive classification efforts to obtain relatively uniform particle sizes.